I think they're an adequate substitute for real noodles, but only in limited contexts. I use them in Asian stir fries in place of rice noodles and don't miss the real thing too much because there are so many other ingredients and flavors. I also make a very low-cal mac and cheese. The cheese is tasty enough that the noodles are of secondary importance. However, I would never find them a satisfying replacement for a dish that centers around high quality pasta (e.g. Italian dishes).
I highly recommend them for their low calorie/high fiber stats, but you may have to experiment to find recipes that work for your tastes. They're quite filling, so at only 20 calories a serving, they're definitely worth a try!